Simple is better than complex.

Lib/this.py

@@ -20,9 +20,4 @@ Vs gur vzcyrzragngvba vf uneq gb rkcynva, vg'f n onq vqrn.
 Vs gur vzcyrzragngvba vf rnfl gb rkcynva, vg znl or n tbbq vqrn.
 Anzrfcnprf ner bar ubaxvat terng vqrn -- yrg'f qb zber bs gubfr!"""
 
-d = {}
-for c in (65, 97):
-    for i in range(26):
-        d[chr(i+c)] = chr((i+13) % 26 + c)
-
-print "".join([d.get(c, c) for c in s])
+print s.decode('rot_13')